1. Field of the Invention
The present invention relates to a device having a light-transmitting electrode and a pad electrode which are formed on a p-type GaN related compound semiconductor layer.
2. Description of the Related Art
In conventional compound semiconductors, an ohmic contract is obtained by depositing metals on the semiconductor surface and heating the metal to convert the same to an alloy and to cause metal diffusion into the semiconductor, because an ohmic contact is not attainable by the mere deposition of metals.
Even when the p-type GaN related compound semiconductors are subjected to a treatment for reducing resistance, e.g., irradiation with electron beams, the thus-treated semiconductors still have higher resistivities than n-type GaN related compound semiconductors. Consequently, in such p-type GaN related compound semiconductors, the p-type layer has almost no current flow in lateral directions, and only the part thereof directly beneath the electrode emitts light.
Under these circumstances, a current-diffusing electrode having light transmission properties and ohmic properties has been proposed which if formed by depositing a nickel (Ni) layer and a gold (Au) layer, each having a thickness of several tens of angstroms (.ANG.) and heating the metal layers (see Japanese Unexamined Patent Publication No. Hei. 6-314822).
However, the electrode formed by depositing nickel (Ni) and gold (Au) each having a thickness of several tens of angstroms and heating metal poses a problem that the light-emitting pattern quality deteriorates with the lapse of time, resulting in an increased driving voltage. However, the electrode has satisfactory optical and electrical characteristics in the initial stage.
The reason for the quality deterioration is believed to be as follows. Since the nickel (Ni) and gold (Au) deposited layers are extremely thin, part of the nickel (Ni) is replaced by gold (Au) during the heat treatment, and the nickel (Ni) exposed on the electrode surfaces oxidizes to for NiO. When current is caused to flow through the electrode in this state, the NiO reacts with the OH.sup.- group of water present in the surrounding atmosphere to form a substrate comprising NiOOH, as shown by the following scheme (1). Since NiOOH has a poor wettability by gold (Au) and by the GaN related compound semiconductor, the NiOOH aggregates. As a result, light-emitting pattern quality deteriorates with the lapse of time and the contact resistance of the electrode increase. Thus, conventional art devices employing the proposed electrode are believed to deteriorate in optical and electrical characteristics. EQU NiO+OH.sup.-.fwdarw.NiOOH+e.sup.- (1)
Further, since this current-diffusing electrode is thin, a pad electrode made of Ni/Au or Au is formed therein for bonding.
However, the conventional art device described above has insufficient adhesion between the pad electrode and the current-diffusing electrode. Hence, if the surface of the current-diffusing electrode on which a pad electrode is to be formed has been soiled, there is a problem that the finally obtained device has problems such as the peeling of the pad electrode and a poor light-emitting pattern. In addition, even if the pad electrode has satisfactory adhesion to the current-diffusing electrode, the light emission occurring in the shade of the bonding pad cannot be directly observed, unavoidably resulting in a light emission loss.
Further, there is still another problem as follows.
In conventional GaN related compound semiconductors, low-resistivity p-type conduction is not attainable by mere doping with a p-type impurity. It has hence been proposed to impart p-type low resistance to a GaN related compound semiconductor doped with a p-type impurity by irradiating th doped semiconductor with electron beams (see Japanese Unexamined Patent Publication No. Hei. 2-257679) or by subjecting the doped semiconductor to thermal annealing (see Japanese Unexamined Patent Publication No. Hei. 5-183189). It has also been proposed to conduct the thermal annealing for imparting p-type low resistance simultaneously with alloying for forming an electrode (see Japanese Unexamined Patent Publication No. Hei. 8-51235).
However, in the method using thermal annealing described in Japanese Unexamined Patent Publication No. Hei. 5-183189, the heat treatment should be conducted at a room temperature not lower than 700.degree. C. in order to obtain a saturated low resistivity. Although this kind of semiconductor has conventionally employed aluminum as the main electrode material, use of a temperature not lower than 700.degree. C. for electrodes alloying produces problems, such as the formation of aluminum balls resulting from aluminum melting, and impaired surface state, increased contact resistance of the electrode, and wire bonding failure.
Consequently, the heat treatment for electrode alloying should be conducted at a relatively low temperature of from 500 to 600.degree. C. It is, however, noted that the heat treatment for imparting p-type low resistance does not result in a sufficiently low resistivity when conducted at a temperature in the range of from 500 to 600.degree. C. It has hence been necessary to conduct the heat treatment for imparting p-type low resistance and the heat treatment for electrodes alloying as separate steps, respectively.
On the other hand, Japanese Patent Publication No. Hei 8-51235 proposes to conduct the impartation of p-type low resistance simultaneously with electrode alloying by performing a heat treatment as a temperature of from 400 to 800.degree. C. However, this method has the following problems. The impartation of p-type low resistance is insufficient in the low-temperature range where electrode alloying is achieved satisfactorily. In the high-temperature region suitable for the sufficient impartation of p-type low resistance, electrode alloying cannot be conducted satisfactorily, resulting in increased contact resistance and poor ohmic properties.